1. Field of the Invention
The present invention relates to a semiconductor device which is provided with a function of controlling a current supplied to a load by a transistor, and more particularly to a semiconductor device comprising a pixel comprising a current drive type light emitting element whose luminance varies with current, and a circuit for supplying a signal to the pixel.
2. Description of the Related Art
As a display device using a self-light emitting element typified by an organic light emitting diode (OLED, which is also referred to as an organic EL element, an electroluminescence (EL) element, and the like), there are two types: a passive matrix and an active matrix according to a driving method thereof. The passive matrix display device has a simple configuration while it has a difficulty in realizing a display with a large size and high luminance. In recent years, an active matrix display device in which a current supplied to a light emitting element is controlled by a thin film transistor (TFT) provided in a pixel circuit has been developed.
The active matrix display device has a problem in that a current supplied to a light emitting element varies due to variations in current characteristics of driving TFTs, leading to luminance variations. That is, a pixel circuit adopts a driving TFT for driving a current supplied to a light emitting element, and thus the current supplied to the light emitting element varies as characteristics of such driving TFTs vary, so that luminance varies. In view of this, various circuits are proposed, in which luminance variations are suppressed without changing a current supplied to a light emitting element even in the case where characteristics of driving TFTs vary among pixel circuits (e.g., refer to Patent Documents 1 to 4).
(Patent Document 1)
Published Japanese translation of PCT international publication for Patent Application No. 2002-517806
(Patent Document 2)
PCT International Publication for Patent Application No. WO01/06484 pamphlet
(Patent Document 3)
Published Japanese translation of PCT international publication for Patent Application No. 2002-514320
(Patent Document 4)
PCT International Publication for Patent Application No. WO02/39420 pamphlet
Disclosed in Patent Documents 1 to 3 are circuit configurations for preventing the fluctuation of a current value supplied to a light emitting element due to variations in characteristics of driving TFTs among pixel circuits. Such a configuration is referred to as a current write type pixel or a current input type pixel. In Patent Document 4, a circuit configuration for suppressing fluctuation of a signal current due to variations of TFTs in a source driver circuit is disclosed.
FIG. 6 shows a first configuration example of a conventional active matrix display device disclosed in Patent Document 1. A pixel in FIG. 6 comprises a source signal line 601, first to third gate signal lines 602 to 604, a current supply line 605, TFTs 606 to 609, a storage capacitor 610, an EL element 611, and a current source 612 for inputting an image signal.
An operation from a signal current writing to a light emission is described with reference to FIGS. 7A to 7E. In FIGS. 7A to 7E, reference numerals denoting respective parts conform to those shown in FIG. 6. FIGS. 7A to 7C schematically show current paths. FIG. 7D shows a relationship between currents flowing through respective paths during the signal current writing. FIG. 7E shows a voltage accumulated in the storage capacitor 610 during the signal current writing, namely a gate-source voltage of the TFT 608.
Firstly, a pulse is inputted to the first gate signal line 602 and the second gate signal line 603 to turn ON the TFTs 606 and 607. A current flowing through the source signal line 601 at this time, namely a signal current is referred to as Idata here.
The current Idata flows through the source signal line 601, therefore, the current separately flows through paths I1 and I2 in the pixel. The relationship between I1 and I2 is shown in FIG. 7D. It is needless to say that Idata=I1+I2 is satisfied.
At the moment in which the TFT 606 is turned ON, a charge is not yet held in the storage capacitor 610, thus the TFT 608 is OFF. Accordingly, I2=0 and Idata=I1 are satisfied. That is to say, only a current due to an accumulated charge in the storage capacitor 610 flows at this time.
Then, charge starts to be accumulated in the storage capacitor 610 gradually to cause a potential difference between both electrodes (FIG. 7E). When the potential difference between both electrodes reaches Vth (point A in FIG. 7E), the TFT 608 is turned ON, generating I2. Since Idata=I1+I2 is satisfied as mentioned above, the amount of I1 gradually decreases while a current continues flowing and charge is accumulated in the storage capacitor 610.
In the storage capacitor 610, the charge accumulation is continued until the potential difference between both electrodes, namely the gate-source voltage of the TFT 608 reaches a desired voltage, that is a voltage (VGS) that allows the TFT 608 to flow a current of Idata. When the current accumulation is completed (point B in FIG. 7E), the current I1 stops flowing and a current corresponding to VGS at this time flows through the TFT 608, thus Idata=I2 is satisfied (FIG. 7B). A steady state is achieved in this manner and a signal writing operation is completed. At the end, the selection of the first gate signal line 602 and the second gate signal line 603 is completed to turn OFF the TFTs 606 and 607. The above operation is referred to as a setting operation here.
Subsequently, a light emitting operation starts. A pulse is inputted to the third gate signal line 604 to turn ON the TFT 609. Since the storage capacitor 610 holds VGS that has been written, the TFT 608 is ON and a current of Idata flows therethrough from the current supply line 605. Accordingly, the EL element 611 emits light. At this time, in the case where the TFT 608 is set to operate in a saturation region, Idata can continue flowing even when the source-drain voltage of the TFT 608 varies.
Such an operation of outputting a set current is referred to as an outputting operation here. By using the current write type pixel, a gate-source voltage required for flowing the current Idata is held in the storage capacitor 610, so that a desired current can be supplied to the EL element 611 accurately even in the case where the TFTs 608 have a variations in characteristics and the like. Consequently, luminance variations due to characteristic variations of TFTs can be suppressed.
The abovementioned example relates to the technology for correcting the change in current due to variations of driving TFTs among pixel circuits, however, the same problem occurs in a source driver circuit. Patent Document 4 discloses a circuit configuration for preventing the change in signal current due to the manufacturing variations of TFTs in a source driver circuit.
(Patent Document 5)
Japanese Patent Laid-open Publication No. 2003-66908
Patent Document 5 discloses a configuration in which a voltage source is additionally provided as well as a current source for controlling gray scale, and a charge of floating capacitance is instantaneously changed by the voltage source at the beginning of a row selection period by a power source switching means for switching two power sources to input to a source signal line, and then a gray scale display is performed by the current source for realizing desired luminance.